13. Ashley Place
My dear Sir—
In 1852 the declination2 at Geneva3 was 18º.2’ W. The secular change4 is not far from 6’.5 a year, westerly diminishing. Consequently in 1857, 5 years later it should be 32’.5 less than in 1852, or about 17º. 30’.
There is no doubt whatsoever that the Declination was West, not East, when Forbes’s map5 was drawn. And it is quite true that the line of Magnetic North6 is set off erroneously in regard to the true Meridian7 in his map. The error may not go further; the survey may be correct as regards the true or Astronomical bearings: and the draftsman’s error in the compass rose8 be the only mistake. If so it is an innocent one—but it may also be much more serious—of this I cannot judge, but you may be able to do so. If some parts have been laid down by astronomical bearings, and some by magnetic bearings with an erroneous declination allowed, the survey would be altogether vitiated—I hope that this is not the case; but that the error may be confined to the Rose.
You may allow 6’ 1/2 as the secular change; it may be 1/2 a minute too little—If you should want greater exactness I can look further into the matter—
Sincerely yours | Edward Sabine. | Oct. 19th
RI MS JT/1/S/15
RI MS JT/1/TYP/4/1308
[19 October 1857]: the date is given based on mention of the year in the first paragraph, and the month and day are written in the signature line.
declination: of the magnetic needle: the deviation from the true north and south line, esp. the angular measure of this deviation (OED).
Geneva: see letter 1293, n. 3.
secular change: in scientific use, of processes of change: having a period of enormous length (OED).
Forbes’s map: ‘Map of the Mer de Glace of Chamouni. And of the Adjoining Mountains laid down from a detailed Survey in 1842 by Professor Forbes’ in J. D. Forbes, Travels through the Alps of Savoy (see letter 1306, n. 8).
Magnetic North: the direction of the north magnetic pole (OED).
true Meridian: terrestrial meridian, or the great circle of the earth which lies in the plane of the celestial meridian of a place, and which passes through that place and the terrestrial poles; (also) that half of the latter circle which extends from pole to pole through the place, corresponding to a line of longitude (OED).
compass rose: a circle on a map showing the principal directions.
Please cite as “Tyndall1444,” in Ɛpsilon: The John Tyndall Collection accessed on